(a) Field of the Invention
This invention relates generally to the manufacture of webs of material, notably polymer films, and specifically to an improved apparatus for, and method for, winding such webs or films so as to obtain coils or reels thereof for storage and/or further processing.
(b) Description of the Prior Art
Continuous winding machines, notably for winding of paper or polymer webs formed or processed continuously in a preceding production or finishing step are well known in the art: cf. U.S. Pat. Nos. 1,687,928, 2,915,255 and 3,494,566, and Swiss Patent No. 540,185.
The feature common to all prior art winders is a mechanism for receiving a substantially endless web or film material and for guiding such film or web onto an empty cylinder or tube (core mandrel) so as to form a coil or reel of the web that can be stored or used as a web-source for further processing, e.g. printing.
Prior art winders for a substantially automatic operation further include a mechanism for replacing web-wound core mandrels (reels) by empty core mandrels so as to enable continued winding, that is, without interrupting the web stream when a reel is discharged and an empty core mandrel is introduced. The operating sequence of such winders starting with the take-up of a leading edge of the moving web by an empty core mandrel and ending with cutting-off the web from the reel with concurrent formation of the next leading edge will be called a "cycle" herein.
Another characteristic feature of conventional winders is the type or mode of operation with regard to rotation of the core mandrel: one group of winder operates in a so-called "center winding" fashion, that is, by connecting the core mandrel with a drive acting substantially centrally upon the core mandrel so that the web is pulled thereon; the other group of winders operates in a so-called "surface winding" fashion, that is, by contacting a generally linear portion of the web surface on top of the core mandrel with a rotating winding drum; the winding drum is operated by drive and the web is "pushed" rather then pulled onto the core mandrel.
Generally, the surface winding fashion or mode of operation is preferred for winding of paper, such as in paper mills; this preference is understood when considering the mechanical properties of paper webs and the advantage of avoiding rupture of such webs due to pulling tension by applying but a "pushing" force that frictionally engages the coil surface.
Most winders in the plastics industry operate according to the surface winding mode as well and many polymer films or webs, such as, typically, polyalkylene films, can be wound on machines for the surface winding mode, either because such films have no or very little blocking tendencies per se or because their inherent blocking tendencies are substantially modified by the use of conventional slip additives.
When attempting to wind polymer films having a substantial blocking tendency on a surface-mode winder, the resulting coils--if they can be obtained at all--tend to lack in smoothness both of the layer structure of the reel or coil as well as of its surface structure. Such lack of smoothness indicates a deformation of the film and such deformed films tend to cause problems upon further processing, e.g. printing.
The blocking tendency of a web or film generally indicates a high coefficient of friction of the film; films with such properties are of growing importance as is the tendency to avoid use of slip additives. As a consequence, additional or supplemental winding machines are required that operate essentially in the center winding mode is smooth and, hence, undeformed layer structures are required for coils of high-friction films and/or those including no slip additive and two different types of winding machines would be, and in fact are, required for winding of different type of polymer films; such duplication of machines for a specific production or processing step is, of course, disadvantageous from an operating point of view, notably because typical steps preceding the winding, such as extrusion, do not generally require different apparatus for production of polymer films that have different frictional coefficients.